Abstract

The short term effect of phosphorus in sewage sludge was investigated to establish howmuch of this a farmer can grant him- or herself, if using sewage sludge as a phosphorusfertilizer. Possible differences in plant availability between sludge phosphorus precipitatedby iron, aluminum or calcium integrations were studied in a sandy soil at two pH levels.

The sludges were analyzed using pot and incubation trials. As the intention was to investigategrowth limitation due to phosphorus deficiency, a soil with low phosphorus supplywas chosen as well as a low phosphorus application rate. Sludge corresponding to 12kg phosphorus per hectare was applied. Treatments fertilized by sludge were compared totreatments where 0, 6 and 12 kg phosphorus per hectare from mineral fertilizer was applied.Every treatment was replicated six times. Three of those replications had initial pH6.2 in soil, whereas three were limed to the achieve pH 7.2. In the pots ryegrass (Loliumperenne) was sown. The pot trial was performed in a greenhouse during 11 weeks.

The yield and uptake of phosphorous was higher in the pots were no lime was applied.A plausible theory to explain that effect may be that phosphorus strongly bound to calciumwhen pH increased. Sewage sludge had under the conditions for this trial a short-term effectof phosphorus corresponding to 20 – 60 % of that from a mineral fertilizer. Iron andaluminum precipitated sludge gave in this trial a higher short-term effect than sludge precipitatedby calcium, probably due to the negative effect on phosphorus uptake from liming.Sewage sludge application elevated ammonium lactate extractable phosphorus (P-AL)to a varying extent. This variation did not correspond to the observed variations of phosphorusfertilizer effect between the sludge types that were examined.